There are periodic
astronomical events that could never be observed by any human alive
today. One of these is the transit of Venus on June 8, 2004.

On this day
half our globe will be able to watch the tiny black dot of the planet
Venus moving across the disc of the sun. Given a cloudless sky, all
you'll need is a sun filter to follow the spectacle for several hours
by your naked eyes (check
your
eyes).

The previous
transit of Venus occurred on December 6, 1882 -
one of merely five events of its kind ever watched by mankind.

Which
conditions have to be fulfilled for a Venus passage ?
Why have Venus transits been of great importance for astronomy ?Which
other planets can show a transit ?